Traditional dyeing processes of the type well known to those skilled in the art require the use of very large volumes of water. The bulk of the water present in these processes (>95%) is used for heating, rinsing, agitation, dissolution of chemicals and dye dispersion. This heavy usage of water naturally has significant environmental implications in view of the limited water resources which are available and the requirement to subsequently treat contaminated waste. Self-evidently, there are also substantial associated cost implications in terms of energy, water and process equipment.
As is well known in the coloration industry, there are vast numbers of processes available for the application of various classes of dyes to very many different fibre types. Typical dye classes include water soluble dyes such as acid dyes, basic dyes, direct dyes and reactive dyes, as well as sparingly water-soluble disperse dyes, and dyes which are solubilised during the dyeing process, for example vat dyes and sulphur dyes. All of these dyes are typically applied to textile fibres in the form of aqueous solutions or dispersions.
Amongst the fibre types coloured by such processes are included natural fibres, such as wool, cotton and silk, and man-made fibres as exemplified by cellulose acetate and lyocell, as well as synthetic fibres, for example polyesters, polyamides such as nylon, polyalkenes and polyacrylonitrile. Various blends of different fibre types, such as polyester/cotton, wool/nylon and polyester/viscose/cotton, are also coloured by such processes, often using processes which employ blends or mixtures of different classes or types of dye for each of the different fibre types present in the blended fibres.
Different conditions (pH, temperature, electrolyte; duration of treatment, liquor ratio, etc.) are currently used for the application of the various classes of dye to the different types of fibre. Amongst the commonest dyeing processes in this regard, there may be mentioned direct dyes on cotton, acid dyes on wool, reactive dyes on cotton and disperse dyes on polyester. This diversity of application method is both historical and necessary owing to the physical and chemical differences between the different fibres, and the different chemical natures of the various dyes. Consequently, markedly different conditions are required to apply the various classes of dyes to the various fibres.
Different finishes (e.g. water repellency, anti-crease, etc.) may also be applied to the dyed materials, again using different methods according to the nature of both the finish and type of fibre. Furthermore, different conditions are frequently required for the application of both dyes and finishes to the (chemically) same textile fibre depending on the particular physical form in which it is processed, including, for example, yarns, hanks, open width fabric, garment, etc.
Furthermore, various chemical pre-treatments are commonly required to prepare textile fibres for dyeing and chemical finishing. Thus, for example, scouring processes are often employed to clean the materials, especially in the case of natural fibres such as cotton and wool, whilst bleaching processes are used to reduce the yellowness of natural fibres, such as cotton, and to impart enhanced levels of whitenesss of the textile material.
As noted above, conventional dyeing methods consume vast volumes of water (typical liquor ratios being in the range of ˜4-20:1 liquor ratio, depending on the type of fibre being treated); in addition, they employ huge quantities of dyeing auxiliaries such as electrolytes, surfactants, alkalis, acids and other such materials and, thereby, generate massive quantities of wastewater which, depending on factors such as the type of dye, depth of shade, fibre type and substrate construction being used, may contain residual dyes, electrolytes, acids, alkalis, and the like, and which can display marked recalcitrance towards biodegradation, thereby presenting both environmental and economic challenges. Indeed, many processes have been developed for the treatment and disposal of dye wastewater, including traditional wastewater treatment methods such as adsorption, electrochemistry and oxidation, as well as nanofiltration, photocatalysis, irradiation and biosorption.
A previous approach to addressing the problem of high levels of water consumption has been the application of supercritical carbon dioxide dyeing techniques, wherein CO2 is heated to temperatures in the region of 120° C. and pressurised at around 20-30 MPa. These conditions cause the CO2 to swell and penetrate the fibres, as well as dissolving the dyes, thereby causing dyeing to take place in ˜60-120 minutes. However, such processes are high consumers of energy and use large volumes of CO2. Furthermore, since the technology is suitable only for non-polar disperse dyes (because polar dyes are insoluble in liquid CO2), it is of no value for the dyeing of wool, cotton, silk, etc., all of which require the use of polar dyes (e.g. the dyeing of cotton with reactive dyes). Such technology is currently under development by DyeCoo (http://www.dvecoo.com/).
An alternative strategy involves the use of ultrasound, which serves to increase dye dispersion and to degas dye solutions, thereby facilitating increased rates of dye diffusion inside textile fibres. Despite the achievement of encouraging results on a laboratory-scale during the last two decades, however, this technology has not yet proved to be attractive on a commercial scale.
A further approach has involved the development of various solvent dyeing techniques, wherein organic solvents are employed as replacements for water, or as co-solvents with water, in order to promote dyeing, especially of synthetic fibres with disperse dyes. This approach has been explored from the 1970s but, despite considerable research interest, the use of organic solvents in dyeing has not achieved commercial success, owing to the obvious concerns relating to the environment, in addition to the lack of economic and technical advantages which are associated with such an approach.
The present inventors have, therefore, sought to develop an approach that allows for significant reductions in the amount of water and dyeing auxiliaries, including various electrolytes, acids, alkalis and surfactants, which are used in the dyeing of substrates, especially textile fibres, and which also avoids the disadvantages associated with the various alternative approaches which have previously been explored. As a consequence, the inventors have succeeded in providing a process that has produced results which are comparable in quality (evenness and wash fastness) to conventional approaches, but which allow for the use of very significantly reduced amounts of water; indeed, water levels are typically reduced to ≤5-10% of the water levels used in conventional processes.
In developing this approach, the inventors have also addressed the issue of wash-off procedures for the treated substrates, most particularly dyed substrates, and have sought to provide wash-off procedures which also allow for the use of very significantly reduced amounts of water, since conventional wash-off procedures typically require the use of copious volumes of water and multiple procedural steps.
Thus, for example, a conventional wash-off process for a reactive dyed substrate would require the use of seven separate wash-baths. The Cyclanon® XC-W New process4 developed by BASF seeks to reduce the number of stages and volumes of water required for the efficient wash-off of reactive dyeings on cotton but, in the case of monochlorotriazine reactive dyes in deep shades, for example, still necessitates the use of five separate baths to achieve satisfactory wash-off. This aspect of dyeing technology is further considered in WO-A-2010/007008, which relates to a process for the washing off of reactive-dyed textile material which comprises a first rinsing step, followed by a dwelling step, which is followed in turn by a second rinsing step. However, the present inventors have sought to still further enhance wash-off processes by providing a method which drastically reduces both the volumes of water and the number of procedural steps which are required.